Addressing South Korean Water Issues in New Ways

Water treatment plant installs country’s first DAF clarification system

The Korean Water Resources Corp. (K-Water) operates the Songjeon Water Treatment Plant in Wonju, South Korea. The plant obtains raw water from the Hoeng-Seong (HS) Lake. The lake water has algae blooms, high turbidity when rainfall events occur and the need for powdered activated carbon (PAC) to enhance organic removals and prevent chlorinated byproducts such as total trihalomethanes (TTHM), dichloroacetic and trichloroacetic acid (HAA2) and chloral hydrate (CH).

The raw water supply from the HS Lake historically had algae (average counts 2455 cells/mL), low turbidity (average 6.1 ntu), low hardness (average 28 mg/L as CaCO3), and low alkalinity (average 21.6 mg/L). From 2000 to 2004, turbidity during and after rainfall events varied to spikes of over 300 ntu. K-Water designed a system that would be flexible and handle a variety of raw water conditions. The plant was designed for flow rates of 100,000 cu m/hr (26.4 mgd). The process treatment scheme included a presedimentation basin, a Leopold Clari-DAF system and Leopold FilterWorx gravity filters with Type S underdrain, I.M.S. (Integral Media Support) cap, with sand (250 mm) and anthracite (1750 mm) media and backwash troughs. The purpose of the presedimentation basin was to divert flow during rain events to allow heavy high-density solids to settle out prior to treatment through the Clari-DAF system flotation process.

The plant was built and commenced operation in 2003. Currently, the coagulant chemical feed is polyaluminum chloride and lime, averaging 12 mg/L and 2 mg/L, respectively. The G value in the flocculators is 70 sec-1 in the first stage and 50 sec-1 in the second stage. The loading rate is 16.3 m/hr (6.7 gpm/sq ft) based on collector area, while the recycle rate is 10% in the summer and 7% during the rest of the year. Since startup of the facility, the average raw turbidity has been 5 ntu, the Clari-DAF system effluent has been 0.3 ntu and the filter effluent has been 0.04 ntu. Rain events have resulted in raw water turbidities ranging from 64 to 430 ntu. During those times, 2.3–16.5 ntu turbidity remained in the effluent from the presedimentation process. This turbidity was reduced by the Clari-DAF system to 0.14–1.1 ntu and then by the filter to 0.02–0.09 ntu, producing a stable water quality.

When removing organics in the raw water, K-Water has elected to reduce the level of chlorinated byproducts below the criteria mandated in Korea and the U.S., as seen in this table:

Table 1

Units : mg/L

Parameter

Korea

USA

K-Water

TTHMs

< 0.1

< 0.08

< 0.064

HAA2

< 0.1

< 0.06

< 0.048

CH

< 0.03

< 0.024

To assist the plant in meeting its goals, PAC was fed prior to the addition of the coagulant. The following table summarizes the average concentrations of chlorinated byproducts in the effluent water with PAC addition from April 2005 to March 2006 and without PAC addition from April 2004 to March 2005:

Table 2

Units : mg/L

Parameter

w/o PAC

w PAC

Goal

TTHMs

0.022

0.022

< 0.064

HAA2

0.029

0.020

< 0.048

CH

0.006

0.007

< 0.024

With PAC, the concentrations of TTHMs and CH were all unchanged, but the HAA2 was reduced by 9 ųg/L. This is significant because, of the three chlorinated byproducts, HAA2 has been the biggest problem in most Korean potable water treatment plants. During this study, the presedimentation basin observed an increase in turbidity when feeding PAC, while the Clari-DAF system showed excellent efficiency for removing turbidity. While feeding PAC, and with raw water turbidity of 3.5 ntu, the presedimentation basin’s average effluent went from 1.9 ntu to 2.8 ntu, while the Clari-DAF system effluent yielded a slight increase from 0.15 ntu to 0.23 ntu. Plant personnel continue to investigate additional ways to further reduce chlorinated byproducts.

Leopold, a Xylem brand

Contact

Nominate

The Water & Wastes Digest staff invites industry professionals to nominate the water and wastewater projects they deem most remarkable and innovative for recognition in the Annual Reference Guide issue. All projects must have been in the design or construction phase over the last 18 months.